1. Field of the Invention
The present invention relates to a prepolymerized catalyst for olefin polymerization, a process for polymerizing an olefin by using the catalyst and a polyolefin produced by the process.
2. Description of the Related Art
Propylene polymers are excellent in the moldability and mechanical strength, and inexpensive and thus widely used as plastics of general purpose. However, depending upon the field of their application, it is desired to improve their properties such as rigidity, transparency and high cycle injection moldability without impairing various characteristics specific to the propylene polymers. It is known that such properties can be improved by increasing the crystallization rate of the propylene polymers.
With respect to additives to improve the crystallization rate, many additives have already been made. Commercially available additives include an aluminum salt of aromatic carboxylic acid, dibenzylidene sorbitol and substituted dibenzylidene sorbitol. These additives (hereinafter, referred to as nucleating agents) is used to improve the crystallization rate while the molten polypropylene is cooled to be a solid state, and thus the spherulite size, which is formed when polymers are crystallized in a spherical form around nucleus and improves the rigidity, is minimized. Therefore, light scattering is reduced, and the improved performance including improvement in transparency and gloss of polypropylene and reduction in molding cycle can be achieved. However, when the nucleating agents are used, shrinkage anisotropy may occur, or depending on molding conditions, the nucleating agents may attach to a die, mold, roll or the like, resulting in a warpage defect. Therefore, the nucleating agent has to be carefully employed according to its uses.
Accordingly, it has been recently suggested that the prepolymerization of vinyl compound is performed to improve the rigidity and transparency of polypropylene. A vinyl saturated cyclic hydrocarbon polymer has a melting point of about 370° C., which is much higher than that of general propylene (melting point of pure isotactic polypropylene is 176° C.), and thus can function as a nucleating agent. Therefore, many studies thereon have been conducted.
For example, JP-S-60-139710 disclosed an addition of nucleating agents, in which a small amount of vinyl saturated cyclic hydrocarbon is polymerized before propylene polymerization. However, there are problems in that the polypropylene powder produced by the method has a greatly reduced bulk density and its productivity is also reduced. Further, the polymerization reactivity of vinyl saturated cyclic hydrocarbon is much lower than those of polypropylene or the like, and even though the polymerization is performed at a high temperature for the purpose of increasing a polymerization rate, the catalytic activity is reduced upon polymerizing propylene. Further, for the purpose of improving the Isotactic index of polypropylene, it has been tried to perform the prepolymerization of vinyl saturated cyclic hydrocarbon in the presence of an external electron donor. However, there is also a problem in that the method reduces the polymerization reactivity of vinyl saturated cyclic hydrocarbon.
Further, JP-H-04-96907 suggested a process for prepolymerizing an olefin being suitable for obtaining a polypropylene having high transparency, in which a multi-stage prepolymerization of olefin is performed in the presence of a titanium compound, an organic aluminum compound and an organic silicon compound, different organic silicon compounds are used in the every prepolymerization stages, and vinyl saturated cyclic hydrocarbon and a styrene compound are also polymerized in at least one stage of the every prepolymerization stages. However, in this method, the polymerization ability of the catalyst is inevitably reduced upon polymerizing propylene.